gallium: remove pipe_index_buffer and set_index_buffer

gallium: remove pipe_index_buffer and set_index_buffer

pipe_draw_info::indexed is replaced with index_size. index_size == 0 means
non-indexed.

Instead of pipe_index_buffer::offset, pipe_draw_info::start is used.
For indexed indirect draws, pipe_draw_info::start is added to the indirect
start. This is the only case when "start" affects indirect draws.

pipe_draw_info::index is a union. Use either index::resource or
index::user depending on the value of pipe_draw_info::has_user_indices.

v2: fixes for nine, svga